Membrane-based gas separation/pervaporation systems offer significant energy savings over other gas separation systems such as distillation, adsorption, cryogenic separation, and the like. Unfortunately, there is often a trade-off between the permselectivity and permeability properties of polymeric membranes so that those membranes with the highest selectivities generally have very low permeabilities.
Attempts have been made to increase the separation factor or selectivities of prior polymeric membranes by incorporating separation selective moieties into the polymer surface. Of the many methods used for making such composite membranes, including irradiation grafting, plasma coating, copolymerization, and the like, plasma processes are the most interesting.
Thin polymeric layers can be deposited onto membrane supports by subjecting the support to reactive or polymerizable species generated in the plasma. The polymerizable species may be generated from conventional vinyl and acrylic monomers as well as their fluorinated derivatives. Monomers containing pendant nitrogen heterocycles (i.e. vinylpyridine, vinyl-imidazole, and vinylpyrrolidinone) are particularly interesting with respect to this invention. Some saturated molecules that do not undergo conventional polymerizations can also be plasma polymerized. Examples include saturated hydrocarbons, halocarbons, amines and fluoromines (e.g. perfluorotributylamine).
Although pinhole-free films can be produced when plasma polymerization is carried out under suitable conditions, the composite membranes thus produced do not possess sufficiently high selectivities for commercial applicability. When the reactive species is 4-vinylpyridine, for example, the resulting thin layer contains a great deal of cross-linking and seldom resembles material polymerized by more conventional routes. In general, if the substrate has a high permeability, then membranes possessing attractive composite properties can be fabricated when a thin layer of a highly selective but low permeability material is plasma-deposited on the surface.
Using existing permeability data, one can choose the most desirable thin layer for plasma-deposition onto a polymeric membrane substrate. Because poly (4-vinylpyridine) and other polymers with pendant nitrogen heterocycles exhibit extremely high selectivity (.alpha.O.sub.2 /N.sub.2 =12.2 for poly(4-vinylpyridine), they would be the thin layer of choice. However, the permeability of 4-vinylpyridine polymers is extremely low (PO.sub.2 =2.8 barrers). Efforts to incorporate the pyridine moiety into polymers known to have good permeability in order to increase their selectivity have not resulted in commercially acceptable membranes.
For example, Lai et al., J. Appl. Polym. Sci., 32, 5763 (1986) describe gamma radiation-induced graft polymerization of a poly(4-methylpentene) substrate soaked in a solution of 4-vinylpyridine. This is a bulk modification that would lead to an interpenetrating polymer network. Likewise, Hsuie et al., J. Appl. Polym. Sci., 32, 4615 (1986), Nishide et al., Japan Chem. Lett., (CMLTAG) (1986), and Sumita, Japanese Patent No. 51-72976 teach bulk modification to incorporate the vinylpyridine moiety into substrate polymers.
On the other hand, Kawakami et al., J. Membrane Sci., 19, 249 (1984) describe the synthesis of composite membranes made from "rubbery" substrates and thin films of plasma polymerized "monomers" such as pyridine, 4-vinylpyridine, 2-vinylpyridine, perfluorotributylamine, vinylpyrrolidinone, and the like. This reference specifically demonstrates that plasma-polymerizing such monomers onto "stiff" supports, such as polysulfones and polyurethanes results in a plasma deposit that is liable to crack or separate from the substrate due to internal stress arising from the formation of cross-linkages and/or oxygenation after deposition. Accordingly, Kawakami et al. teach that only substrates of high flexibility, such as natural rubber or silicone rubbers, should be used as a substrate. However, highly flexible, hence soft and rubbery substrates such as polydimethylsiloxanes (the best polymer disclosed by Kawakami et al.) are not sufficiently permeable to be commercially attractive. A higher permeability material is required.